I don’t have strong opinions about the Reversible class because I don’t imagine needing it. My instinct was __reverse__() is independent of __iter__(), so it should not be a subclass. But I don’t really mind either way.

I did actually mean a version changed notice for the data model change. I see this as a small expansion of the Python object API. Previously, __reversed__() had to be a function, now you are also allowed to set it to None. The collections ABCs are just catching up with the API change. Imagine someone using an Orderable virtual base class that tested for __gt__() etc.

If you need, you can write repetitive tests without copying and pasting or generated code:

for [param, result1, result2] in parameters:
    with self.subTest(param=param):
        ...

Maybe it is okay to add a test to ABCTestCase.validate_isinstance() to check that None cancels isinstance().

I also added some Reitveld comments for patch4a.